Servo systems of disk drives control a radial position of a transducer coupled to the disk in an attempt to keep the transducer over the center of a pre-selected track of the disk. In a disk file system this is done as the transducer reads position information from the track in the surface of the disk as the disk rotates. For an embedded or sectored servo system the position information is obtained from inter sector information placed at predetermined locations on the tracks of the disk. The position information is then used to develop a position error signal (PES). The PES is fed back through a compensator into a drive motor for the transducer actuator to move the transducer in a direction to reduce the error from the desired position within any given track. Typically, the desired position is the geometric center of the track with the PES value increasing as a positive value in one direction from the center and as a negative value in the other.
The servo systems include a feedback servo loop (head actuator loop). The PES is coupled into the head actuator loop and is the primary means by which the head actuator is maintained in the desired position over the center of a selected track. Although the aforementioned feedback servo loop in combination with the PES can adequately maintain the position of the head actuator in most use situations, mechanically induced disturbances or displacements of the disk drive, however, cannot be totally eliminated because of the finite response time and finite gain of the feedback servo loop correcting for these disturbances or displacements. A finely balanced mechanical actuator can be used to desensitize rotary actuator disk drives to translational disturbances by means of directing the translational disturbances solely through the center of the pivot point of the actuator. This creates a zero length moment arm about the pivot point which results in no net torque or angular forces to disturb the actuator to push it out of position when a purely translational disturbance is applied. This is true to a first order as slight imbalances, the stiffness of actuator connector wires, and pivot bearing stiffness contribute to some translational sensitivity, but this is relatively small in comparison to the sensitivity to rotational disturbances. However, in order to access data, the actuator must be able to pivot freely with minimal bearing friction. Because of this, the actuator is susceptible to rotary disturbances. Such disturbances either rotate the head away from the track center or rotate the track center away from the head. Either of these results in a net increase in the position error signal (PES) sensed by the read/write head. Such effects result in the unreliable reading and writing of data to and from the desired tracks on a disk drive. Thus, when the head is forced sufficiently off track—typically from 8% to 15% of the track width—the reading and writing of data is discontinued.
Several important trends within the data storage industry have made the effects of rotational disturbances increasingly severe wherein a cost effective solution is more important than ever before. The rise in availability, density, and cost effectiveness of Electrically Erasable Programmable Read Only Memory (EEPROM) has hastened the need to be able to add increased storage capacity to disk drive products with no room for commensurate cost or physical size increases. Thus the materials used to construct all critical components of a disk drive, from storage platters to actuators and mechanical structures must be both lower cost, and often lighter as well. Such design requirements generally lead to less rigidity and more susceptibility to mechanical disturbances that are only partially mitigated by the shrinking of the drive dimensions. Also, the traditional means of generating more storage capacity on disk drives has been to pack more tracks of data onto disks of the same or smaller size. More tracks on a disk leads to a higher number of Tracks Per Inch (TPI) and thus increasingly narrow track widths which makes accurate alignment over track centers significantly more challenging for an unassisted servo system. Since EEPROM is an entirely semiconductor-based technology, it does not suffer from issues related to mechanical disturbances and so, to be competitive, disk drives must also mitigate its performance disadvantages due to mechanical disturbances as well.
With the improvement in read/write heads and the commensurate increase in areal densities the requirement for tracking accuracy is constantly increasing. With finer track widths comes an increased susceptibility to both internal and external disturbances. While improving the drive feedback loop can improve susceptibility to issues that include—but are not limited to—spindle harmonics, PES noise, and disturbances due to air flow, the finite sample rate of the control system and the system resonances limit the bandwidth of the closed-loop system. Thus, for external disturbances, especially rotational shock and vibration, auxiliary sensors are often necessary to provide the needed disturbance rejection and maintain tracking fidelity.
Because most disk drives use the sectored servo method of generating the PES, there is a direct trade-off between the space allocated for user data and that allocated to record position information. Thus, to increase the performance of the servo control loop solely by means of adding more PES sectors comes at a direct cost to the available space for user data storage.
Finally, two of the largest markets for data storage are mobile computing (including laptops, portable communications, gaming and navigation devices) and corporate data centers. The susceptibility of mobile disk drives to the mechanical disturbances of shocks and bumps are fairly obvious. However, the requirements of corporate datacenters are for applications that run the gamut from corporate databases (for enterprise management software to regulatory document storage for audit compliance) to search engines, to video sharing internet sites. The efficiency of these centers is directly tied to the ability to constantly read and write data to the disk drives in the center. It is in these high volume data storage applications where multiple disk drives are ganged into server boxes where the effects of mutual coupling of disk drive operations such as seeks and spindle wobble create disturbances that cause severe degradation in the throughput of its neighboring drives. Finally, as disk drive track densities increase, the issues of shock and vibration become more significant for the commodity drives in desktop and laptop computers as well.
One possible solution includes the use of a monolithic rotational accelerometer to sense rotational shock and vibration of the disk drive. The rotational accelerometers generate a signal which can be used as a feedforward controller to the servo loop for making the disk drive more robust to shock and vibration. However, because of the continual push to reduce the manufacturing costs of disk drives, monolithic rotational accelerometers are impractical for such a cost sensitive, high volume application due to their relatively high cost. Monolithic rotational accelerometers are relatively expensive because they are extremely difficult to manufacture. This is true for all such sensors in general, but especially so for MEMS rotational accelerometers as they are fabricated on silicon wafers. Because rotational accelerometers seek to provide a perfectly balanced output that is immune to translational forces, their construction is very difficult. For the MEMS variety, the additional complexity involved in the masking and etching process of the silicon wafers used in their fabrication creates asymmetries which cause all but a few of the fabricated accelerometers to exhibit non-balanced outputs susceptible to translational accelerations. There are analogous difficulties involved in the fabrication of the non-MEMS varieties of rotational accelerometers as well.
In addition, rotational accelerometers suffer from poor sensitivity because the acceleration sensing structures are in such close proximity to each other. This is problematic because rotation is best sensed along the largest possible diameter about a given center of rotation. Finally, rotational accelerometers require factory calibration in order to provide a known gain in response to a specific rotational input as well as to provide true rejection of linear acceleration. This calibration step also adds significant cost to the devices and also becomes a significant production bottleneck when large volumes must be produced.
Another type of sensor which can be employed is a single-axis linear-translation accelerometer, both of MEMS and non-MEMS varieties. These sensors are designed to be sensitive to only linear acceleration along their axis of sensitivity and do not respond to angular acceleration when the axis of rotation passes through the center of the sensor. They are much less expensive and simpler to fabricate than their rotational counterparts. It is well known in the art that two linear accelerometers can be configured to measure rotational accelerations by arranging them parallel to each other on opposing tangents of a circle of rotation about a desired point of rotation. When arranged in this manner the difference of their signals yields pure rotation while the sum yields pure translation. If the sensors are arranged in an anti-parallel configuration, then the sum of their signals yields rotation while their difference yields translation. The terms “combination” or “combining” are occasionally used to mean either a difference or a sum, or the act of their calculation, which yields the desired result of either rotation or translation depending upon the specific polarities of the sensor outputs wherein a number of factors could impact the net polarity of said sensor outputs.
The advantage of using linear accelerometers for rotational acceleration measurement is that they are low cost and also, depending upon the pair's separation distance, can create relatively large signal output levels in response to a rotational acceleration. This greatly enhances their usability, especially when the rotational stimulus is small.
In order for linear accelerometers to be used effectively for angular acceleration measurements in the paired configuration, their respective gains must be well matched, otherwise there is an incomplete separation between rotational and translational acceleration components. The offsets of the accelerometers can be easily compensated for by the electrical drive circuitry, and therefore, are not addressed. Even the most expensive and well-produced linear accelerometers typically vary in gain by up to +/−15% from sensor to sensor. The gain mismatch between low cost sensors can be even greater. This leads to significant errors in measured angular acceleration when using such linear accelerometers, especially when there is a presence of translational acceleration as well. In addition to the actual sensor gain mismatches, there is a significant influence to measured accelerations depending upon how and where the sensors are actually mounted to the disk drive. For instance, if one sensor is mounted closer to a housing feature, such as a screw or a mounting boss, the transfer of acceleration to the measuring sensor can be either amplified or attenuated compared to its partner. This means that even if a pair of linear accelerometer sensors were perfectly calibrated at the factory prior to installation into the disk drive, by the time they are mounted, their effective gains would again be mismatched.
It is desirable to have a low-cost, easy to implement method and apparatus for compensating for disturbances of a disk drive.